At present, memory cells which are constituted by integrated injection logic are elements of paramount importance, because such memory cells can be formed with high integration density. Such memory cells are disclosed, for example, in the articles "Write Current Control and Self Powering in a Low Power Memory Cell", IEEE, SSC, June, 1973, and "Superintegrated Memory Shares Functions on Diffused Islands", Electronics, Feb. 14, 1972, p83-p86. The former discloses a method of controlling a write current to the integrated injection logic memory cell and the latter discloses a basic idea with respect to the integrated injection logic memory cell.
As disclosed in the above mentioned articles, the integrated injection logic memory cell comprises a pair of first and second transistors which have emitters forming an injector which is connected to a word line W+, and said first and second transistors have a first conductivity type, a pair of third and fourth transistors which have collectors respectively connected to the collectors of the first and second transistors, the base of the third transistor being connected to the collector of said fourth transistor and the base of the fourth transistor being connected to the collector of said third transistor, and which have a second conductivity type, and a bulk that is a word line W- which is connected to the bases of the first and second transistors and to the emitters of the third and fourth transistors.
The integrated injection logic memory cells are arranged as the memory array. In these integrated injection logic memory cells, the word line W- is formed as the bulk, as already mentioned, and this bulk usually consists of two n type layers, an epitaxial layer and a buried layer. The epitaxial layer is formed on the buried layer which has a higher density of the impurity than the epitaxial layer. Therefore, the bulk, that is, the word line W- has a larger resistance than a metallic wire, and this resistance exists between each cell.
When the bulk is used as the word line W- which supplies the hold current, the characteristics of the cells are different in accordance with the positions of the cells in the line of the array. Especially, a write threshold current and the width of a write pulse have large values in cells near the end of the array.